Friends keep asking me to explain the loss of EgyptAir 804, but of course I am just as confused as everyone else.
The only thing that I learned from my airliner-flying days (sample) that might be relevant is that most of the important stuff in an airliner is under the floor. I still wonder “Why doesn’t a passenger spilling a Diet Coke take out half of the radios?”
The flight deck door is now armored but the floorboards under the bathroom aren’t. So in theory a malicious passenger could perhaps open a hole in the bathroom floor and put an incendiary device into the avionics bay. This would explain the “lavatory smoke” message that was received and also result in a catastrophic problems for a fly-by-wire Airbus A320. Perhaps a malicious passenger could shortcut the process by pouring explosive/flammable stuff down the drain and lighting it on fire? I don’t know enough about the plumbing in an A320 to make an educated guess about that. (I do know that the Ryanair proposal to charge passengers to use the bathroom made a lot of sense; it is hard to think of anything more costly than putting tanks of liquid into the pressure vessel of an aircraft.)
It is sad that 66 people died to leave us this puzzle, but the puzzle yet remains.